1. Field of Invention
The present invention generally relates to a field of mobile communication, and in particular to a method and apparatus for generating a PDU and a base station thereof, which may be applied in the Single Frequency Network (SFN) to recover multiple packets lost during the transmission of service data so as to ensure contents synchronization between different base stations.
2. Description of Prior Art
In a wireless network transmission system adopting a SFN technology, for example, in a Multimedia Broadcast and Multicast (MBMS) system of LTE, all base stations (hereinafter referred to as eNB) totally keep synchronous in both time and frequency. In this way, if each eNB transmits the same data at one starting point and adopts the same physical layer modulation and coding technologies, in the space transmission, each signal sent from respective eNBs has the same waveform. At a user equipment (hereinafter referred to as UE) side, data sent from the eNBs may be considered as a signal sent from one eNB. Since signals received by the UEs are all useful signals carrying the same data, such a technology may significantly reduce interferences coming from neighboring cells (because signals coming from the neighboring cells here are all useful signals, rather than interferences), thus may be applied in some broadcast and multicast systems. For example, such a SFN technology may be applied in the MBMS system of LTE.
FIG. 1 shows a network topology for the LTE MBMS. Network elements supporting MBMS mainly include Broadcast/Multicast Service Center (BM-SC), SAE GateWay (SAE GW), eNB and UE. In general, an interface between a UE and an eNB is called as S1 interface.
As an entry for contents provider, BM-SC is adapted to perform an authorization, initiate a MBMS bearer service in the PLMN, and transmit MBMS data in accordance with a preset schedule. SAE GW controls the MBMS bearer service for the user, and transmits the MBMS data to E-UTRAN. The eNB is responsible for transmitting the MBMS data to an air interface of a designated MBMS service area with high efficiency.
FIG. 2 shows a schematic diagram of a network structure for the SFN. In FIG. 2, each base station is covered with the same frequency point, and it is unnecessary to perform time multiplexing, frequency multiplexing, or code multiplexing between cells. One service is assigned with the same time and frequency resources in any area covered by the base stations. Since the same physical layer modulation and coding technologies are applied, each signal carrying the same data sent from respective base stations has the same waveform. In view of the receiver of the UE, the signal looks like to be sent from one base station, just with some multi-paths having large time delay. The SFN may be fully covered with normal transmission powers of all base stations or improved transmission powers of a part of base stations.
In the LTE wireless communication system, the wireless interface protocol layer is divided into three layers. The bottom layer is Physical Layer (L1), above which a Data Link Layer (L2) and Network Layer are involved. At the wireless interface, the Data Link Layer is divided into several sub-layers, including a Media Access Control (MAC) protocol sub-layer, a Radio Link Control (RLC) protocol sub-layer, and a Packet Data Convergence Protocol (PDCP) sub-layer. The Network Layer (L3) provides Radio Resource Control (RRC) function.
In the whole protocol, IP data packets arrive at respective eNBs, and each IP data packet has a variable length, for example, ranging from tens of byes to 1500 bytes. Furthermore, since there are different paths to respective eNBs, multiple packets loss will occur during the transmission. Therefore, it is critical to ensure the contents synchronization for the data sent from respective eNBs for one eMBMS service. In view of this, it is needed to achieve a simple recovery for the eNB wireless processing when there are packets lost during the data distribution to the eNBs.
Here, packet loss recovery means filling pseudo data into a wireless link control buffer without recovering real data in the packet, since the contents synchronization for the transmissions of respective packets can be ensured after the packet loss as long as the eNB knows the length of the packet to be transmitted.
FIG. 3 shows a relationship between RLC SDU (Service Data Unit) and RLC PDU (Protocol Data Unit) in the existing unicast situation, in which each segmentation has a length indicator. As shown in FIG. 3, it is assumed that, at the eNB side, the RLC PDU has a size of 600 bytes, two bytes for SN and SI are included in the RLC PDU header, and each LI has a size of 2 bytes.
It can be seen from FIG. 3 that two packets are lost at the S1 interface (SN=i, i+1). When the eNB receives a packet numbered as SN=i+3, it is known that the previous two packets have been lost. However, the eNB only knows that there are two packets lost and the total length of the packet is of 600 bytes, it does not know the length distribution of the 600 bytes in the lost two packets.
FIG. 4 shows a distribution of byte length for the two lost packets. As shown in FIG. 4, one of the lost two packets is of 400 bytes, the other one is of 200 bytes, and they are divided, in together, into three segmentations, each of which is indicated using respective LIs in different PDUs. For example, in the previous PDU, two LIs respectively indicate 296 bytes from the SDU numbered as SN=i and 298 bytes from the SDU numbered as SN=i+1. In the posterior PDU, the three LIs respectively indicate 102 bytes from the SDU numbered as SN=i+1, 200 bytes from the SDU numbered as SN=i+2, and 290 bytes from the SDU numbered as SN=i+3. If recovery is performed following such a manner, the starting byte of the next PDU will be 1286.
FIG. 5 shows another distribution of byte length for the two lost packets. As shown in FIG. 5, one of the lost two packets is of 298 bytes, the other one is of 302 bytes, and they respectively serve as one segmentation, thereby forming two segmentations in total, each of which is indicated using respective LIs in different PDUs. For example, in the previous PDU, two LIs respectively indicate 296 bytes from the SDU numbered as SN=i and 298 bytes from the SDU numbered as SN=i+1. In the posterior PDU, the two LIs respectively indicate 302 bytes from the SDU numbered as SN=i+2 and 292 bytes from the SDU numbered as SN=i+3. If recovery is performed following such a manner, the starting byte for the next PDU will be 1288.
It can be seen from the above examples that the de-synchronization for the RLC PDU contents will be incurred since the number of segmentations for the recovered RLC SDU numbered as SN=i+1 is different from the number of segmentations for the recovered RLC SDU numbered as SN=i+2, and each segmentation has respective LI, for example there are two LIs or three LIs.
Consequently, since segmentations of a single SDU have respective LIs, the eNB can not know the distribution of byte length for respective packets in case of multiple packets loss, and the lost packets can not be correctly recovered, thus leading to the contents de-synchronization at the eNB. Moreover, the contents de-synchronization would affect contents synchronization of subsequent packets. In a word, using the unicast RLC PDU format can not recover lost packets at the eNB but easily leading to the contents de-synchronization.